The field of the invention is Radio Frequency (RF) transponders (RF Tags) which receive RF electromagnetic radiation from a base station and send information to the base station by modulating the load of an RF antenna.
RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the objects, items, and people. It has been known since the early 60's in U.S. Pat. No. 3,098,971 by K. M. Richardson, that electronic components on a transponder could be powered by (RF) power sent by a “base station” at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current led to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna could be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own it is called a “passive” tag to distinguish it from an active tag containing a battery. The battery supplies energy to broadcast the information from the tag antenna. An active tag may also change the loading on the tag antenna for the purpose of transmitting information to the base station.
The “rebroadcast” of the incoming RF energy at the carrier frequency is conventionally called “back scattering”, even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed “back” to the transmitting antenna.
In the 70's, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor. A transistor switch also changed the loading of the transponder in U.S. Pat. No. 4,786,907 by A. Koelle.
A combination diode rectifier circuit and balanced modulator for modulating the antenna current at twice the carrier frequency was proposed by Gary T. Carroll in U.S. Pat. No. 4,724,427.
Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station.
The continuing march of semiconductor technology to smaller, faster, and less power hungry has allowed enormous increases of function and enormous drop of cost of such tags. Presently available research and development technology will also allow new function and different products in communications technology. The use of the prior art transistor switches to change the loading of the transponder antenna and to receive information, however, leads to increased cost in the use of a totally integrated system consisting of a single chip connected to an antenna. The transistor switch of the prior art must be fast enough and have low capacitance to work well contained on a chip in a reasonable time. Such transistors lead to increased costs in the chip manufacturing, as the entire chip must be made with the same technology and the entire chip does not need the speed of the one transistor element. The range of the communication distance from the base station to the tag is critical. This range is determined by the voltage built up by the antenna and rectifying circuits on the tag. Passive RF tags must do two things which are incompatible. First, there must be a steady supply voltage extracted from the modulated RF field to power the devices on the tag. Second, there must be a data signal recovered from the modulated RF field which has well defined zeros and ones for use by the tag digital electronics. If the signal is taken off from the voltage on the main power supply capacitor of the tag, the voltage swing must be low to provide good power for the electronics, and high to provide good signal.
The information receiving sections of prior art RF tags draw down the main power supply capacitor which supplies power to the tag when no RF power is sent from the base station. This is wasteful of energy and useless, since there is no information to be received when the RF power is off.
Prior art tags have modulating circuits and receiver circuits which reduce the voltage which can be produced by the rectifier circuits. Prior art tags have circuits which require relatively high current, which reduces the voltage built up by the antenna and rectifying circuits on the tag.